Free convection cooling method and apparatus

ABSTRACT

A system of heat transfer utilizing free convection flow of a coolant to lower the temperature of a circulating heated transport fluid, a heat exchanger submerged in a coolant bath being constructed and arranged to obtain the greatest possible temperature-density differential between the circulating transport fluid and environmental coolant productive of strong natural convection currents.

United States Patent [72] Inventor Raymond S.De Groote 1,029,631 Centerville, Ohio 268,126 21 Appl. No. 728,412 1,937,713 [22] Filed May 13, 1968 1,940,338 [45] Patented Jan. 19, 1971 2,277,094 [73] Assignee United Aircraft Products, Inc. 2,469,028 Dayton, Ohio 2,820,617 a corporation of Ohio 3,225,824 3,289,757 [54] FREE CONVECTION COOLING METHOD AND APPARATUS 1 1,232,985 3 Claims, 3 Drawing F igs. [52] US. Cl 165/1, 126/366,165/107,165/108,165/131,165/132, 165/153 51 1m. (:1 ..F28d 15/00, F28d 1/02 [50] Field of Search 165/108, 128, 132, 176, 166, 153, 131, 107,01; 126/366 lant bath [56] References Cited UNITED STATES PATENTS 111,721 2/1871 Blabe 165/128 tion flow of a coolant to lower heated transport fluid, a heat Primary Examiner-Albert W. Davis, Jr. Attorney-J. E. Beringer ABSTRACT: A system of heat transfer utilizing free convecthe temperature of a circulating exchanger submerged in a coobeing constructed and arranged to obtain the greatest possible temperature-density differential between the circulating transport fluid and tive of strong natural convection currents.

environmental coolant produc- FREECONVECTION COOLING METHOD AND 1 1- APPARATUS I .1 T his invention relates to a method. of=andmeans of heat transfer, and particularly to a concept of cooling by induced natural convectionflow. r

lna commonly encountered heat. transfer system, a fluid absorbsheateither as. a result of circulating to and from a heat source, as for example functioning electronic equipment, or in consequence of its use, as for example in hydraulic actuating circuitsgPrior to returning the heated fluid-for reuse, it. is put through .a heat exchanger. Excess heat is-dissipated. The tem-' perature of the: returningfluid is limited to a selected high value; Inithe heat exchanger the heated fluid is brought into heat transfer relation taacoolant and it is customary to flow theco'olant or cooling fluid through the heat exchanger under pressure. Thus, ina conventional system the heat exchanger is a focal point for heated fluid, which may be termed the transport fluid, andia cooling fluid, both-pumped through the heat.

exchanger under pressure. The arrangement allows a wide selection of heat exchanger tjypesand configurations. Moreover, compact, small size heat exchangers arepossible since-pressure differences are available to 'make advantageous use of velocityeffects and circuitry for maximum heat transfer efficiency.- j" 1 j I In the event forced. flow in the coolant side of the heat transfer system is present'in unusually low flow rates, and 7 pressure values heat exchanger options'are reduced; The size of the heat exchanger must be substantially increased to provide greater heat transfer surface. ln'the absence of any forced coolant flow, the heat transferr'equirement, that is, the desired rate of heat absorption from the circulating transport fluid,

must be satisfied by natural or free convection flow of a' coolant avenue heattransfer surfaces; De vices designed to forced'convection parameters may notmeet the required performance in the 'natural convection mode. 'Further, a-heat exchanger designed to "meet such "performance requirements mayhave a size and configuration, necessary to provide adequate heat transfersurface, making it economically im- I Referring to FIG. I, a system in accordance with the illus- I from the heat exchanger 12 by supply andreturn lines 15 and 16 respectively. A pump 17 provides for forced flow of the transport fluid. A valve 18 is interposed in the lines 15 and 16 tions of the flowing transport fluid to the end that at least some ofthe circulating fluid is bypassedlwhen rising pressure indicates blockage of heat exchanger passages or when a lowering temperature indicates the transport fluid to. be not in need of cooling. The transport fluid is shown circulated through a heat source 19, this being a diagrammatic representation of means by which the transport fluidis c aused to' absorb heat. As will be understood, there may be anactual heat source 19 in the form of an engine or electronic package or the like with respect to which the heat transport fluid'is passed in heat absorbing relation. Alternatively, smay designate a reservoir of hydraulic oil drawn upon for use in hydraulic .actuating systems and heated thereby. In any event, transport fluid heated by use and exposure at heat source 19 is picked up by I pumpl7 andfdelivered underpressure through flow line 15 to inlet manifold 3 im heat exchangerl2. Flowing through the. heat exchanger, in a manner to be more particularly considered, the transport fluid reaches outlet manifold 14 and is returned by line 16 to thelheat source The liquid of bath 10 has a temperaturelower thanthat at which transportfluid is delivered to heat exchanger l2. The temperature differential is utilized to cool or. to remove excess heat-from the transport fluid. The construction and arrangement of parts is one to limit the high-temperature value atwhich the transport fluid can be returned for reuse at the heat source.

The heat exchanger 12 is constructed to provide a large amountof heat transfer surface and yet maintain a sufficiently small and compact size as to adapt it for use in a liquid bath of lel plates 21 held in a separated relation at their ends by clopassages 'of;the transport fluid through the heat exchanger 7 being designed to induce strong natural convection currents in the environmental fluid.

In carrying out such objects,'a heat exchanger is submerged in a'tank of relatively .cool liquid, the liquid servingas the coolant and having natural convection flow currents produced therein by circulation of the heated transport fluid through the heat exchanger. The created currents rise through vertical open flow passages in the heat exchanger. Further, horizontal flow passages in the heat exchanger for flow of the transport fluid are arranged to flow such fluid in cross-counterflow relation to the liquid currents. Temperature differentials between the heat transfer surfaces and the upwardly rising liquid coolant are more uniformly distributed. Naturalconvection flow is increased and overall temperature differential is improved.

Other objects and details of the invention will appear from the following description, when read in connection with the accompanying drawings,wherein: 1 i

FIG. I is a diagrammatic view of a system in accordance with the illustrated embodiment of the invention;

FIG. 2 is a view in perspective, partly broken away, of a heat exchanger as shown in FIG. 1; and g FIG. 3 is a fragmentary detail view of a core section of the heat exchanger of FIG. 2.

sure inserts 22. Inaltemating relation to the inserts 22, and closing side marginsof the plates 21, are longitudinally extending insert spacers 23. The-latter are narrow relatively to the inserts22. The arrangement isone to provide open chambers or passages 24 and 25, the former being wide relatively to the latter. In the passages 24 is undulating fin strip material 26 providing secondary heat transfer surface, as well as serving a purpose in lending strength and rigidity to the assembled heat exchanger. Otherstrip fin material 27 is in the flow passages heat exchanger, is a separator strip 32. The strips 32 are positioned midway between the top and bottom of the heat exchanger and at their one ends are coextensive with that end prises a first strip component 33 located above the separator 32 and extending lengthwise of the heat exchanger to terminate in an angled end projecting beyond the inner end of the separator. A like, opposing strip 34 is below the separator whilethe two are in effect joined by a strip segment 35 beyond the separator serving to guide flowing fluid from one longitudinal strip to the other.

A convenient method of making the heat exchanger is to assemble it ina fixture, stacking pans one upon another in the indicated relation. The fixture with contained and assembled parts is then subjected to a brazing or like operation in which the several individual components of the heat exchanger are united one with another. The resulting structure is an integrated device having no movable or separable parts. In a final assembly step, the manifolds l3 and 14 are secured, as by welding, to what may be considered the front end of the heat exchanger. Margins of the manifolds are joined to solid portions of the heat exchanger core, as represented by the ends of plates 2829, insert spacers 22,23, and the ends of separators 32. The latter define a separation point between the manifolds, with portions of passages 25 above the separators communicating with inlet manifold 13 and portions of passages 25 below the separators communicating with outlet manifold 14. The manifolds have respective connector bosses 36 and 37 respectively by which flow lines 15 and 16 may be connected thereto. Disclosure means 31 may have the form of parts brazed integrally with other components, or, if weld I metal, formed by welding after the brazing step.

In the operationof the system, heated fluid from source 19 is pumped to inlet manifold 13. There it has simultaneous communication with the several flow passages 25 above separators. 32. The fluid enters the upper flow passages 25, flows longitudinally therein along fin strips 33 to the opposite end of the heat exchanger where it turns, guided by strip fin segments 35, and returns through lower passages 25 to manifold 14. From there it is conducted by line 16 back to source 19. In the installation of the heat exchanger it is placed in tank 11 in an intermediate position therein and so oriented as to dispose flow passages 24 vertically of the tank and flow passages 25 horizontally thereof. Vertical flow passages 24 are open at their topand bottom so that liquid in the bath 10 has free access to occupy and to flow through such passages. The plates 21, and connecting parts, tend to rise in temperature, in response to circulation of the heated transport fluid through passages 25. The heated plate surfaces defining passages 24, and extended surface as represented by strip fin material 26, conducts heat into the liquid occupying passages 24. The warmed liquid rises from passages 24 and is replaced by cooler liquid from below the heat exchanger. The process is a continuous one, during circulation of the heated fluid, and there is created thereby convection flow currents in the liquid bath 10 substantially as indicated in FIG. I, carrying out continuous cooling of the circulating transport fluid.

The construction and mode of operation of the heat exchanger has been devised with multiple purposes in mind. It provides a large amount of heat transfer surface in a relatively small space. Also, it provides for a uniform distribution of temperature differential between the heat transfer surfaces and the flowing liquid in the liquid bath. Accordingly, liquid flow is substantially uniform over the area of the heat exchanger. Full utilization of the heat exchanger core is made and convection currents of maximum quantity flow are ensured. The flow configuration within the heat exchanger has particular value in obtaining the desired uniform temperature differential. The transport fluid when at a maximum temperature flows through upper flow passages 25 and accordingly is in heat transfer relation to liquid in passages 24 which has already been somewhat warmed by passing through the lower part of such passages. correspondingly, transport fluid at lower temperature values flows through the lower passages 25 and is there in contact with liquid of minimum temperature. Nowhere in the heat exchanger are conditions created in which transport fluid of maximum temperature encounters coolant of minimum temperature. Hot spots, or isolated locations of maximum temperature differential, which may produce localized heavy convection currents with little or no convection flow in other portions of the heat exchanger, are thus avoided.

The heat transfer system hereof has been shown at least in part in a diagrammatic form since many details of the heat exchange apparatus and flow control mechanisms are not essential to an understanding of the invention. It is not intended, moreover, that structural details illustrated and described should havea limiting effect since it is clear that alternate forms exist within the spirit and intent of the invention. 'In general, a preferred embodiment of the invention has been disclosed, but is intended that the scope of the invention should be limited only by the claims allowed herein.

Although not so limited, the invention in its illustrative embodiment is adapted for aircraft use, specifically to cool the hydraulic fluid of one of the hydraulic actuating systems with which modern aircraft are equipped. Thecoolant or heat sink fluid is engine fuel. Forced flow in the fuel circuit being under some conditions unavailable, the heat exchanger is mounted in a fuel tank. The heat transfer requirement is satisfied, considering only natural or free convection induced fuel flow. The embodiment does not contemplate boiling, nor a change of state in either the hydraulic fluid or the engine fuel. The max imum temperature difference therebetween is relatively low, yet sufficient in view of the structure and orientation of the heat exchanger to accomplish the desired cooling.- Engine fuel is not consumed in operation of the system. Cooling is merely a function of strong natural convection currents created in the fuel tank by the uniquely arranged and presented surfaces of the heat exchanger. 1

Iclaim:

1. In a system positively circulating a transport fluid for heat absorption purposes, a method of removing excess heat from such fluid by natural convection circulation of a heat sink liquid not consumed in the heat transfer process, including the steps of locating a heat exchanger in a tank containing a relatively cool heat sink liquid, completely submerging the heat exchanger in the contained liquid beneath the surface of the liquid in said tank and orienting it so that liquid is free to occupy and pass through vertical open flow passages therein, and of flowing the transport fluid through relatively narrow passes in said heat exchanger under forced circulation in heat transfer relation to liquid in said passages, convection currents of flowing liquid being thereby created in said tank flowing upward through said vertical flow passages, the transport fluid being put through said heat exchanger in a circuitous path taking it sequentially in heat transfer relation to upper and lower parts of said vertical flow passages, said heat exchanger providing vertically superposed horizontal flow passages for the transport fluid in alternating adjacent relation to said verti cal flow passages and defining said relatively narrow passes, said horizontal flow passages being of substantially greater length than said vertical flow passages, upper and lower horizontal flow passages communicating with one another at one side of the heat exchanger and being separated at the op posite side of the heat exchanger for in flow and out flow of the transport fluid, in-flowing transport fluid first entering upper horizontal flow passages for crossflow through the heat exchanger to said one side thereof then counterflowing to lower horizontal flow passages and then cross flowing in the opposite direction to exit from the heat exchanger at the said other side thereof.

2. In a system of cooling a positively circulated heated liquid utilizing a free convection mode in which a heat exchanger is submerged in a bath of heat sink liquid, a tank containing heat sink liquid at a temperature lower than the temperature of said heated liquid but not sufficiently different to vaporize when brought in heat transfer relation to said heated liquid so that the heat sink liquid is not consumed in operation of the system, a heat exchanger for use therein fully submerged in said tank beneath the surface of the heat sink liquid, said heat exchanger being of the plate and fin type constructed for cross counterflow wherein open relatively wide flow passages from end to end of the heat exchanger provide for free flow of the heat sink liquid therethrough and confined more narrow flow passages from side to side of the heat exchangerprovide for flow of the heated liquid in transverse heat transfer relation to said open flow passages, said confined flow passages being manifolded for series flow from one side to the other across one end of the heat exchanger and return flow from said other side to the said one side across the other end of said heat exchanger, said heat exchanger having inlet and outlet connections for the heated liquid at'said one side of the heat exchanger, the crossflow passages beingin communication at the said other side of the heat exchanger, said heat exchanger having a flat shallow configuration .in which the width from side to side is substantially greater than the length from end to end. and said heat exchanger being so oriented in the tank that convection currents of heat sink liquid passing through said 

1. In a system positively circulating a transport fluid for heat absorption purposes, a method of removing excess heat from such fluid by natural convection circulation of a heat sink liquid not consumed in the heat transfer process, including the steps of locating a heat exchanger in a tank containing a relatively cool heat sink liquid, completely submerging the heat exchanger in the contained liquid beneath the surface of the liquid in said tank and orienting it so that liquid is free to occupy and pass through vertical open flow passages therein, and of flowing the transport fluid through relatively narrow passes in said heat exchanger under forced circulation in heat transfer relation to liquid in said passages, convection currents of flowing liquid being thereby created in said tank flowing upward through said vertical flow passages, the transport fluid being put through said heat exchanger in a circuitous path taking it sequentially in heat transfer relation to upper and lower parts of said vertical flow passages, said heat exchanger providing vertically superposed horizontal flow passages for the transport fluid in alternating adjacent relation to said vertical flow passages and defining said relatively narrow passes, said horizontal flow passages being of substantially greater length than said vertical flow passages, upper and lower horizontal flow passages communicating with one another at one side of the heat exchanger and being separated at the opposite side of the heat exchanger for in flow and out flow of the transport fluid, in-flowing transport fluid first entering upper horizontal flow passages for crossflow through the heat exchanger to said one side thereof then counterflowing to lower horizontal flow passages and then cross flowing in the opposite direction to exit from the heat exchanger at the said other side thereof.
 2. In a system of cooling a positively circulated heated liquid utilizing a free convection mode in which a heat exchanger is submerged in a bath of heat sink liquid, a tank containing heat sink liquid at a temperature lower than the temperature of said heated liquid but not sufficiently different to vaporize when brought in heat transfer relation to said heated liquid so that the heat sink liquid is not consumed in operation of the system, a heat exchanger for use therein fully submerged in said tank beneath the surface of the heat sink liquid, said heat exchanger being of the plate and fin type constructed for cross counterflow wherein open relatively wide flow passages from end to end of the heat exchanger provide for free flow of the heat sink liquid therethrough and confined more narrow flow passages from side to side of the heat exchanger provide for flow of the heated liquid in transverse heat transfer relation to said open flow passages, said confined flow passages being manifolded for series flow from one side to the other across one end of the heat exchanger and return flow from said other side to the said one side across the other end of said heat exchanger, said heat exchanger having inlet and outlet connections for the heated liquid at said one side of the heat exchanger, the crossflow passages being in communication at the said other side of the heat exchanger, said heat exchanger having a flat shallow configuration in which the width from side to side is substantially greater than the length from end to end, and said heat exchanger being so oriented in the tank that convection currents of heat sink liquid passing through said open flow passages are brought into heat transfer relation to the heated liquid in inverse relation to the order in which the heated liquid passes through said confined crossflow passages.
 3. A system according to claim 2, characterized in that the said open flow passages are of such short length as to obviate an increase in temperature of the flowing heat sink liquid therein approaching the temperature of the heated fluid, the crossflow passages for said heated fluid being substantially longer. 